The invention relates to a process for producing a cold-rolled strip or sheet of steel with good deforming properties, which is subjected to recrystallizing annealing and, if appropriate, dressing operation after hot rolling, coiling and cold rolling and has a bake-hardening potential after a subsequent deformation and for a subsequent temperature treatment.
The invention also relates to a cold-rolled strip or sheet with good deforming properties which can be produced by the process, With a bake-hardening potential after a subsequent deformation and for a subsequent temperature treatment (BH2 potential).
In automobile construction, for example, there is a need for easily deformable sheets, which must be formed relatively thin in order not to allow the Freight of the vehicle to become too great. Sheets of steel of this type are generally produced in the form of a strip, in that a steel slab is cast, hot-rolled and coiled at a certain intermediate temperature. After cooling or the coiled strip to essentially ambient temperature, the strip is cold-rolled to the final thickness. To eliminate the stresses occurring thereby within the material, a recrystallizing annealing is carried out. Subsequently, the strip is generally gently rolled again with a degree of deformation between approximately 0.5 and 2% (dressing).
The easy deformability of the steels is fundamentally at odds with an increase in the strength values of the steel grade, since the increased strength is accompanied in principle by an impairment of the easy deformability. Higher-strength steel grades (for example ZStE and ZStEi), which in spite of higher strength values can be deformed relatively well, have been developed. Steel grades of this type are known, for example, as ZStE from steel-iron material sheets SEW 093 and 094 and as isotropic steel ZStEi, while the conventional xe2x80x9csoftxe2x80x9d steel grades are known as St12 to St15 (corresponding to DC01, DC03, DC04, DC05 in accordance with DIN EN 10130). The steel grades differ here with regard to the addition of microalloying elements and with regard to how the process is conducted. A special steel of this type is, for example, the isotropic steel ZStEi, as described in DE 38 03 064 C2, EP 0 400 031 B1 or DD 285 298 B5, the disclosure of which is incorporated as part of this description.
For many steel grades, there is the possibility of combining good deformability with an increased yield strength after production, by producing the steel with what is known as a bake-hardening potential. The bake-hardening affect has the effect that, in a temperature treatment or the steel, as performed for example during the stove-enamelling of vehicle body sheets, a strengthening is brought about, that is an increase in the yield strength This is an artificial aging of the steel, which brings about the additional increase in strength. The increase in strength is consequently achieved after the deformation of the sheet for creating the desired component has been carried out, with the result that the increase in strength does not have any adverse effect on the deformation of the sheet. It has been found that prior deformation of the sheet influences the bake-hardening affect. The bake-hardening effect brought about only by the temperature treatment, without prior deformation, is indicated as the BH0 value, while a measure or the bake-hardening effect after a deformation has been performed is the BH2 value, which indicates the increase in strength after a deformation of the sheet by 2% on account of a subsequent temperature treatmentxe2x80x94standardized at 170xc2x0 C. for 20 minutes.
The bake-hardening effect is based on a content of dissolved carbon in the steel which lies above the state of equilibrium. To produce this supersaturation of the steel with dissolved C atoms, the recrystallization annealing is carried out after the cold rolling with a continuous annealing furnace. The increase in temperature in the continuous annealing furnace causes carbon to go into solution. Since the sheet is only heated up briefly in the continuous annealing furnace, a temperature distinctly above A1 is used for the recrystallization. The rapid cooling of the steel strip has the effect of producing the fraction of dissolved C atoms, which is several orders of magnitude above the state of equilibrium.
If, on the other hand, the annealing of the coiled steel strip is carried out in the bell-type furnace, i.e. for a comparatively long time, and the associated slow cooling is performed in air, the steel strip remains in the state of equilibrium, with the result that no aging potential (bake-hardening potential) occurs if the carbon content is xe2x89xa70.02%. Only when there are lower carbon contents, which can be set only by complex vacuum treatment, can an aging potential be produced, since it is only with difficulty that the C atoms in solution lead to an iron carbide precipitation (cementite), on account of their low density and the associated longer diffusion paths, and therefore part remains supersaturated in solution, For C contents of xe2x89xa70.02%, the precipitation of the carbon takes place when there is slow cooling, with the result that no dissolved carbon is available for the aging potential. The temperature treatment causes the carbon atoms in the solution to diffuse into dislocation regions of the matrix. This causes the dislocations to be blocked, with the result that an increased amount of stress is required to produce a plastic flow in the material again. This effect is increased considerably by prior deformation of the steal strip supersaturated with dissolved C The deforming operation, for example by deep drawing, leads to a significant increase in the dislocation density. In the case of the temperature treatment, as performed for example in stove enamelling, the carbon atoms diffuse into the dilated regions of the dislocations. In practice, therefore, the bake-hardening effect is relevant after a prior deformation (characterized by BH2).
Depending on the degree of deformation, the forming carried out on the sheets leads to a cold hardening (work hardening). For the use or the bake-hardening steels, the overall strength, obtained from the cold hardening resulting from the forming and the bake hardening resulting from the temperature treatment, is relevant. The known bake-hardening steels, which are produced with a continuous annealing furnace, have an approximately constant yield-strength profile for the sum of the work hardening and bake hardening over the degree of prestraining as a variable. The bake-hardening effect is therefore scarcely relevant in cases of relatively great strain, on account of the highly predominantly cold-hardening component. It is therefore known that the use of bake-hardening steels is predominantly of interest for components of large surface area which undergo only slight forming operations, such a for example mud guards, engine bonnets, car doors and roofs.
It is also known that the bake-hardening effect increases with the content of dissolved atoms up to a saturation value. An excessive content of dissolved C atoms leads to a lack of aging resistance of the steel sheet during age hardening. For bake-hardening steels, therefore a content of dissolved carbon of between 5 and 10 ppm is regarded am optimal.
The restriction of use of the bake-hardening effect to non-vacuum steels which have undergone recrystallizing annealing in a continuous annealing furnace leads to considerable restrictions on the production of suitable steel sheets. It has therefore not been possible in the past to produce advantageous properties of steel sheets, which preferably require recrystallizing annealing in bell-type annealing furnaces, such as for example the production of steel sheets with planar isotropy or quasi isotropy, with a bake-hardening effect.
The invention is therefore based on the problem of making possible the production of strips or sheets of steel of the type mentioned at the beginning with a bake-hardening potential which does not have the conventional restrictions.
To achieve this object, a process of the type mentioned at the beginning is characterized according to the invention in that the recrystallizing annealing is carried out in a bell-type furnace while coiled and in that the strip or sheet is subjected to cooling at a cooling rate of xe2x89xa71xc2x0 C./s after the recrystallizing annealing from a temperature T of 200xc2x0 C.xe2x89xa6Txe2x89xa6A1.
This process according to the invention consequently allows the production of a bake-hardening steel strip or sheet which has undergone recrystallizing annealing in a bell-type furnace, preferably while firmly coiled, to be precise even if the C content in the steel is xe2x89xa70.02%.
In a surprising way, it is possible by the brief annealing according to the invention after the cooling of the recrystallizing-annealed strip or sheet to xe2x89xa6150xc2x0 C., preferably to approximately room temperature, to bring C precipitated as carbides back into solution. Since the temperature of the brief annealing lies below the A1 temperature of the steel, the technological properties of the steel are not otherwise significantly changed, in particular its texture, by this annealing. On account of the brief annealing and the subsequent cooling, which may be performed in the customary way with air but also with water, part of the dissolved C remains in solution and leads to the aging potential for the subsequent temperature treatment, for example during stove-enamelling.
The brief annealing is preferably brought about in a continuous annealing furnace. To produce an adequate bake-hardening effect, at a low annealing temperature T a relatively long annealing period must be maintained, while higher annealing temperatures considerably reduce the annealing period required. It is therefore preferred to use a temperature T of the brief annealing of xe2x89xa7450xc2x0 C. It is also preferred to set the annealing period of the brief annealing to between 2 minutes and 5 minutes.
It will generally be advisable to dress the strip or sheet after the brief annealing, in other words to deform it gently in the customary way. It may also be advisable if the strip or sheet has already been dressed before the brief annealing, although this does not always appear to be required.
For the production of galvanized sheets or strips, it is particularly expedient to use hot galvanizing of the sheet or strip at least as part of the brief annealing. However, the process according to the invention may also be used for sheets which are not to be galvanized at all or galvanized electrolytically i.e., without the effect of heat.